a history of science-4-第22节

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sion　and　make　it　germinal　of　other　discoveries。　And　when　Schwann　put　forward　the　explicit　claim　that　〃there　is　one　universal　principle　of　development　for　the　elementary　parts；　of　organisms；　however　different；　and　this　principle　is　the　formation　of　cells；〃　he　enunciated　a　doctrine　which　was　for　all　practical　purposes　absolutely　new　and　opened　up　a　novel　field　for　the　microscopist　to　enter。　A　most　important　era　in　physiology　dates　from　the　publication　of　his　book　in　1839。


THE　CELL　THEORY　ELABORATED

That　Schwann　should　have　gone　to　embryonic　tissues　for　the　establishment　of　his　ideas　was　no　doubt　due　very　largely　to　the　influence　of　the　great　Russian　Karl　Ernst　von　Baer；　who　about　ten　years　earlier　had　published　the　first　part　of　his　celebrated　work　on　embryology；　and　whose　ideas　were　rapidly　gaining　ground；　thanks　largely　to　the　advocacy　of　a　few　men；　notably　Johannes　Muller；　in　Germany；　and　William　B。　Carpenter；　in　England；　and　to　the　fact　that　the　improved　microscope　had　made　minute　anatomy　popular。　　Schwann's　researches　made　it　plain　that　the　best　field　for　the　study　of　the　animal　cell　is　here；　and　a　host　of　explorers　entered　the　field。　The　result　of　their　observations　was；　in　the　main；　to　confirm　the　claims　of　Schwann　as　to　the　universal　prevalence　of　the　cell。　The　long…current　idea　that　animal　tissues　grow　only　as　a　sort　of　deposit　from　the　blood…vessels　was　now　discarded；　and　the　fact　of　so…called　plantlike　growth　of　animal　cells；　for　which　Schwann　contended；　was　universally　accepted。　Yet　the　full　measure　of　the　affinity　between　the　two　classes　of　cells　was　not　for　some　time　generally　apprehended。

Indeed；　since　the　substance　that　composes　the　cell　walls　of　plants　is　manifestly　very　different　from　the　limiting　membrane　of　the　animal　cell；　it　was　natural；　so　long　as　the；　wall　was　considered　the　most　essential　part　of　the　structure；　that　the　divergence　between　the　two　classes　of　cells　should　seem　very　pronounced。　　And　for　a　time　this　was　the　conception　of　the　matter　that　was　uniformly　accepted。　But　as　time　went　on　many　observers　had　their　attention　called　to　the　peculiar　characteristics　of　the　contents　of　the　cell；　and　were　led　to　ask　themselves　whether　these　might　not　be　more　important　than　had　been　supposed。　　In　particular；　Dr。　Hugo　von　Mohl；　professor　of　botany　in　the　University　of　Tubingen；　in　the　course　of　his　exhaustive　studies　of　the　vegetable　cell；　was　impressed　with　the　peculiar　and　characteristic　appearance　of　the　cell　contents。　He　observed　universally　within　the　cell　〃an　opaque；　viscid　fluid；　having　granules　intermingled　in　it；〃　which　made　up　the　main　substance　of　the　cell；　and　which　particularly　impressed　him　because　under　certain　conditions　it　could　be　seen　to　be　actively　in　motion；　its　parts　separated　into　filamentous　streams。

Von　Mohl　called　attention　to　the　fact　that　this　motion　of　the　cell　contents　had　been　observed　as　long　ago　as　1774　by　Bonaventura　Corti；　and　rediscovered　in　1807　by　Treviranus；　and　that　these　observers　had　described　the　phenomenon　under　the　〃most　unsuitable　name　of　'rotation　of　the　cell　sap。'　Von　Mohl　recognized　that　the　streaming　substance　was　something　quite　different　from　sap。　He　asserted　that　the　nucleus　of　the　cell　lies　within　this　substance　and　not　attached　to　the　cell　wall　as　Schleiden　had　contended。　He　saw；　too；　that　the　chlorophyl　granules；　and　all　other　of　the　cell　contents；　are　incorporated　with　the　〃opaque；　viscid　fluid；〃　and　in　1846　he　had　become　so　impressed　with　the　importance　of　this　universal　cell　substance　that　be　gave　it　the　name　of　protoplasm。　Yet　in　so　doing　he　had　no　intention　of　subordinating　the　cell　wall。　The　fact　that　Payen；　in　1844；　had　demonstrated　that　the　cell　walls　of　all　vegetables；　high　or　low；　are　composed　largely　of　one　substance；　cellulose；　tended　to　strengthen　the　position　of　the　cell　wall　as　the　really　essential　structure；　of　which　the　protoplasmic　contents　were　only　subsidiary　products。

Meantime；　however；　the　students　of　animal　histology　were　more　and　more　impressed　with　the　seeming　preponderance　of　cell　contents　over　cell　walls　in　the　tissues　they　studied。　　They；　too；　found　the　cell　to　be　filled　with　a　viscid；　slimy　fluid　capable　of　motion。　To　this　Dujardin　gave　the　name　of　sarcode。　　Presently　it　came　to　be　known；　through　the　labors　of　Kolliker；　Nageli；　Bischoff；　and　various　others；　that　there　are　numerous　lower　forms　of　animal　life　which　seem　to　be　composed　of　this　sarcode；　without　any　cell　wall　whatever。　The　same　thing　seemed　to　be　true　of　certain　cells　of　higher　organisms；　as　the　blood　corpuscles。　　Particularly　in　the　case　of　cells　that　change　their　shape　markedly；　moving　about　in　consequence　of　the　streaming　of　their　sarcode；　did　it　seem　certain　that　no　cell　wall　is　present；　or　that；　if　present；　its　role　must　be　insignificant。

And　so　histologists　came　to　question　whether；　after　all；　the　cell　contents　rather　than　the　enclosing　wall　must　not　be　the　really　essential　structure；　and　the　weight　of　increasing　observations　finally　left　no　escape　from　the　conclusion　that　such　is　really　the　case。　But　attention　being　thus　focalized　on　the　cell　contents；　it　was　at　once　apparent　that　there　is　a　far　closer　similarity　between　the　ultimate　particles　of　vegetables　and　those　of　animals　than　had　been　supposed。　Cellulose　and　animal　membrane　being　now　regarded　as　more　by…products；　the　way　was　clear　for　the　recognition　of　the　fact　that　vegetable　protoplasm　and　animal　sarcode　are　marvellously　similar　in　appearance　and　general　properties。　The　closer　the　observation　the　more　striking　seemed　this　similarity；　and　finally；　about　1860；　it　was　demonstrated　by　Heinrich　de　Bary　and　by　Max　Schultze　that　the　two　are　to　all　intents　and　purposes　identical。　Even　earlier　Remak　had　reached　a　similar　conclusion；　and　applied　Von　Mohl's　word　protoplasm　to　animal　cell　contents；　and　now　this　application　soon　became　universal。　　Thenceforth　this　protoplasm　was　to　assume　the　utmost　importance　in　the　physiological　world；　being　recognized　as　the　universal　〃physical　basis　of　life；〃　vegetable　and　animal　alike。　This　amounted　to　the　logical　extension　and　culmination　of　Schwann's　doctrine　as　to　the　similarity　of　development　of　the　two　animate　kingdoms。　Yet　at　the；　same　time　it　was　in　effect　the　banishment　of　the　cell　that　Schwann　had　defined。　　The　word　cell　was　retained；　it　is　true；　but　it　no　longer　signified　a　minute　cavity。　　It　now　implied；　as　Schultze　defined　it；　〃a　small　mass　of　protoplasm　endowed　with　the　attributes　of　life。〃　This　definition　was　destined　presently　to　meet　with　yet　another　modification；　as　we　shall　see；　but　the　conception　of　the　protoplasmic　mass　as　the　essential　ultimate　structure；　which　might　or　might　not　surround　itself　with　a　protective　covering；　was　a　permanent　addition　to　physiological　knowledge。　The　earlier　idea　had；　in　effect；　declared　the　shell　the　most　important　part　of　the　egg；　this　developed　view　assigned　to　the　yolk　its　true　position。

In　one　other　important　regard　the　theory　of　Schleiden　and　Schwann　now　became　modified。　　This　referred　to　the　origin　of　the　cell。　Schwann　had　regarded　cell　growth　as　a　kind　of　crystallization；　beginning　with　the　deposit　of　a　nucleus　about　a　granule　in　the　intercellular　substancethe　cytoblastema；　as　Schleiden　called　it。　But　Von　Mohl；　as　early　as　1835；　had　called　attention　to　the　formation　of　new　vegetable　cells　through　the　division　of　a　pre…existing　cell。　Ehrenberg；　another　high　authority　of　the　time；　contended　that　no　such　division　occurs；　and　the　matter　was　still　in　dispute　when　Schleiden　came　forward　with　his　discovery　of　so…called　free　cell…formation　within　the　parent　cell；　and　this　for　a　long　time　diverted　attention　from　the　process　of　division　which　Von　Mohl　had　described。　All　manner　of　schemes　of　cell…formation　were　put　forward　during　the　ensuing　years　by　a　multitude　of　observers；　and　gained　currency　notwithstanding　Von　Mohl's　reiterated　contention　that　there　are　really　but　two　ways　in　which　the　formation　of　new　cells　takes　placenamely；　〃first；　through　division　of　older　cells；　secondly；　through　the　formation　of　secondary　cells　lying　free　in　the　cavity　of　a　cell。〃

But　gradually　the　researches　of　such　accurate　observers　as　Unger；　Nageli；　Kolliker；　Reichart；　and　Remak　tended　to　confirm　the　opinion　of　Von　Mohl　that　cells　spring　only　from　cells；　and　finally　Rudolf　Virchow　brought　the　matter　to　demonstration　about　1860。　　His　Omnis　cellula　e　cellula　became　from　that　time　one　of　the　accepted　data　of　physiology。　This　was　supplemented　a　little　later　by　Fleming's　Omnis　nucleus　e　nucleo；　when　still　more　refined　methods　of　observation　had　shown　that　the　part　of　the　cell　which　always　first　undergoes　change　preparatory　to　new　cell…formation　is　the　all…essential　nucleus。　Thus　the　nucleus　was　restored　to　the　important　position　which　Schwann　and　Schleiden　had　given　it；　but　with　greatly　altered　significance。　　Instead　of　being　a　structure　generated　de　novo　from　non…cellular　substance；　and　disappearing　as　soon　as　its　function　of　cell…formation　was　accomplished；　the　nucleus　was　now　known　as　the　central　and　permanent　feature　of　every　cell；　indestructible　while　the　cell　lives；　itself　the　division…product　of　a　pre…existing　nucleus；　and　the　parent；　by　division　of　its　substance；　of　other　generations　of　nuclei。　The　word　cell　received　a　final　def